Harald Pauli

Aim: The European alpine life zone is a recognized biodiversity hotspot, currently impacted by warming temperatures. Here, we investigated temporal plant diversity changes and thermophilization processes on multiple summits' flora. Location: Italy: six mountain regions across the Alps and the Apennines. Methods: We evaluated two decades (2001–2024) of plant species presence and abundance across 23 summits, covering an elevation gradient from the treeline ecotone to the nival belt, monitored through permanent sampling areas. We investigated temporal trends of species gain and disappearance, species richness, alpha and beta diversity indices, species‐specific abundance changes, and the thermic indicator of plant assemblages. Results: Three summits in the Central and Northern Apennines experienced net species loss, involving endemic taxa, although species richness significantly increased in all other summits. At summit level, 7% of species significantly declined in abundance (’losers’), and with the same rate across the upper alpine and subnival belts (7%). About 5% of the species significantly increased (’winners’), and colonization rates were highest in the uppermost vegetation belts. The thermic indicator significantly increased in all mountain regions but the Central Apennines, with increasingly higher rates from the lowermost to uppermost vegetation belts, and accelerated during 2015–2024 in the Texel Group. Main Conclusions: We provide evidence of recent increasing thermophilization trends across the Alps and the Northern Apennines. The local plant species loss, including endemic taxa, and the widespread trend of loser species (not yet disappeared) may signal future local extinctions across both the Alps and Apennines also at the high‐elevated belts. The colonization rate observed in the uppermost vegetation belts further suggests a shift towards warmer thermic conditions at high elevations.

Climate change increases plant species richness in alpine ecosystems. However, to what extent this diversity enrichment masks extinction dynamics of resident species remains elusive. In this study, we used floristic resurvey data from 896 permanent vegetation plots across 62 European mountain summits to show that local extinctions have increased over the past 21 years. Extinction rates rose with the magnitude of warming, and species were more likely to go extinct toward their low-elevation range margins and in communities undergoing stronger thermophilization. Moreover, local extinctions were significantly related to preceding abundance declines, which can serve as an early warning signal. These findings suggest that despite increasing plant species richness, plant assemblages above the treeline face an accelerating but so far neglected loss of their most characteristic species.

Recent changes in alpine vegetation are often attributed to climate warming, particularly community composition shifts towards more warmth-associated species, or thermophilization. Here we assess the link between thermophilization and warming across 53 European mountain summits. We combine long-term macroclimatic and microclimatic temperature time series with vegetation surveys in 724 permanent plots, monitored over 21 years, to evaluate a possible thermophilization signal and relate it to rates of change in 10 temperature metrics. We find evidence of both thermophilization of alpine plant communities and an increase in temperatures. However, although these two trends are related when averaged across mountain regions, their relationship is weak at the individual plot scale, especially when considering microclimatic temperature metrics. Instead, substrate conditions and particularly the availability of thermophilic colonizers in the surrounding vegetation have a major influence on plot-level thermophilization rates. We conclude that the response of plant communities to climate change strongly depends on the abiotic and biotic context, and intensified monitoring efforts are needed to reduce the resulting uncertainties.

Climate change is driving significant changes in alpine ecosystems, where temperature‐sensitive plant communities are particularly vulnerable. As plants migrate to higher elevations, understanding the mechanisms behind these changes is critical for predicting ecosystem dynamics. Morphological, physiological, and chemical functional traits may help understand species' responses to environmental change. This study examines whether functional traits can explain recent changes in presence and cover of alpine plant species in permanent plots. We analyzed vegetation data from repeated surveys of permanent plots at two sites. Morphological traits (e.g., plant height, leaf area) and physiological traits (e.g., frost and drought resistance) were measured locally for common species and supplemented with data from the TRY trait database to broaden species coverage. Linear models were applied to assess the relationships between traits and abundance changes (presence and cover), using multiple metrics. Our results reveal that plant height and leaf area are significant predictors of species abundance changes, emphasizing the role of morphological traits in shaping alpine plant communities. In contrast, physiological traits showed limited explanatory power. Notably, leaf carbon content emerged as a key predictor, suggesting that conservative strategies may provide advantages under warming conditions. Measures describing biomass dynamics (e.g., cover) differed from those describing establishment (e.g., presence), highlighting the multifaceted nature of species responses to environmental changes. Although our findings emphasize the importance of competitive interactions and resource acquisition, the study is limited by the absence of traits related to heat tolerance and prolonged warming. Future research should address these gaps to better understand the impacts of sustained temperature increases on alpine ecosystems.

Shrubs are expanding across the cold ecosystems of our planet with potentially profound consequences for their biodiversity and functioning. However, evidence is still strongly biased towards the Arctic tundra, while a large‐scale assessment of shrub expansion in alpine areas above the elevational treeline is missing so far. Here we quantified shrub cover changes over the past two decades in 576 permanent plots of 1 m ² spread across the alpine vegetation belt of Europe's major mountain chains. Total shrub cover clearly increased in the plots with an average rate of about 2.6% per m ² per decade (95% CI = 1.9%–3.4%), and this expansion was more pronounced for evergreen (2.0% per m ² per decade, CI = 1.3%–2.7%) than for deciduous species (1.7% per m ² per decade, CI = 0.9%–2.4%). The magnitude of individual species' cover shifts was positively associated with their plant height, but negatively with their leaf nitrogen content and light affinity. In sum, we show that shrub expansion is a widespread phenomenon also in the alpine zone of European mountains, with potentially far‐reaching consequences for alpine plant dynamics, soil microclimates, snow patterns, carbon cycling, food chains and livelihoods.

The increasingly acknowledged and consequently also better understood microclimatic variability in terrestrial ecosystems has motivated a call for finer spatial resolution in species distribution modelling, especially in the case of sedentary low‐stature organisms such as plants. In contrast, less attention has so far been paid to the way climate should be represented in these models. In fact, most modelling applications rely only on a handful of so‐called bioclimatic variables (i.e. essential climatic variables designed for ecological applications), which is at odds with the hypothesised variation in the sensitivity of individual plant species to different facets of climate. We argue that the recent shift towards microclimate modelling provides a window of opportunity for re‐evaluating the predominant reliance on the small set of bioclimatic variables. We used a unique dataset of 895 1‐m ² plots with vascular plant species observations and in situ soil temperature measurements across a high‐mountain landscape spanning 1700 elevational metres. From the hourly temperature measurements, we calculated bioclimatic variables as well as 188 additional ‘grid variables' arising from an aggregation of various summary statistics over different parts of the year. We then used those ‘grid variables', their subsets, and the bioclimatic variables to fit species distribution models for 101 plant species with combinations of one, two or three predictors. We found that bioclimatic variables consistently delivered less accurate models than many of the grid variables. Models based on subsets of grid variables only slightly decreased in accuracy and remained superior over bioclimatic variables even after the set was narrowed from the initial 188 to only six variables on the basis of a cluster analysis. These results highlight that modelling species distributions with only a few climatic variables is a viable strategy. However, the most suitable variables may often be different from those that are commonly used nowadays.

Aim: Understanding the resilience and adaptability of alpine flora under climate change is crucial for biodiversity conservation. While functional traits are key to predicting alpine plants' responses to climate change, the role of regeneration traits remains underexplored. We hypothesised that alpine species thriving under climate change produce seeds with higher dispersal ability, longer soil persistence, lower dormancy requirements, and faster germination, while declining species would show opposite traits. Location: Twenty‐three summits across six mountain ranges in Central and Southern Europe: Sierra Nevada, Northern and Central Apennines, and Northeastern, Central, and Southern Alps. Methods: We analysed long‐term data on frequency and abundance changes and eight seed traits related to dispersal, establishment, and soil persistence for 177 alpine species using linear mixed‐effect models. Results: Over two decades, alpine plant populations remained stable, with nearly 90% of species showing minimal frequency change and 70% showing minimal abundance change. However, abundance shifts varied by region: 16%–25% of species declined in Sierra Nevada, the Central Apennines, and the Southern Alps, while the Northeastern Alps and the Northern Apennines showed the largest increases (27% and 17%, respectively). Significant but limited relationships between seed traits and population dynamics were captured, primarily in the Central and Northern Apennines. Species with lower potential for epizoochory or anemochory were more likely to increase in abundance, while smaller seeds were linked to ‘winners’ in some regions. Germination traits, such as broader temperature requirements and slower germination, characterised species with increased abundance in the Northern Apennines. Main Conclusions: Seed traits had limited predictive power in distinguishing ‘losers’ and ‘winners’ of climate change among European alpine plants. This likely reflects the longevity of alpine plants, short observation periods, and potential mismatches between seed‐level microenvironmental conditions and broader climatic trends.

Besides environmental sorting, other processes like biotic interactions and dispersal limitation are vital for the assembly of plant communities in high mountains and their re‐assembly under changing climatic conditions. Nevertheless, studies that compare the impact of these factors on plant community assembly above the tree line are largely lacking so far. We analysed occurrence changes in vascular plant communities of 492 permanent 1‐m² plots in the alpine‐nival ecotone of Mt. Schrankogel, Austrian Alps by comparing resurvey data from 2014 with data from the initial survey in 1994. We combined these data with species inventories from 899 additional plots sampled in 2021 and 2022 across a larger landscape above the tree line covering an elevational range of 1700 m, which we used for fine‐scale habitat suitability modelling. We assessed the relative effects of projected habitat suitability, propagule pressure from surrounding populations and biomass density of neighbours on 1532 colonization and 372 extirpation events of 31 species observed on the permanent plots. We found that all three factors are significantly related to both colonisations and extirpations, with habitat suitability having the strongest, propagule pressure a slightly weaker, and vegetation density the weakest effect. Colonisations can be better explained by the three process proxies than extirpations. Our results indicate a crucial role of dispersal limitation besides the predominant effect of environmental filtering on the (re‐)assembly of the alpine‐nival plant community, while competitive/facilitative effects between plants tend to play a minor role. The strong imprint of nearby source plant populations on colonization/extirpation events suggests that recent plant migrations predominantly occur in small steps. This implies that while the topographically complex alpine terrain offers climatic microrefugia for plants, it may also pose potential barriers, hindering species from following their suitable climatic niches upwards. Synthesis: Besides filtering by environmental conditions dispersal limitation had a strong effect on the observed changes in a local alpine plant community over two decades. Limited dispersal capacities of plant species may counteract the ability of isolated cold areas to effectively shelter high alpine plants from the effects of climate warming.

Mountain regions harbor unique and rich biodiversity, forming an important part of our global life support system. This rich biodiversity underpins the ecological intactness and functioning of mountain ecosystems, which are imperative for the provision of key ecosystem services. A considerable amount of data are required to assess ecological intactness and ecosystem functioning and, given the profound anthropogenic pressures many mountain regions are being subjected to, are urgently needed. However, data on mountain biodiversity remain lacking. The essential biodiversity variables (EBVs) framework can help focus efforts related to detecting, investigating, predicting, and managing global biodiversity change, but has not yet been considered in the context of mountains. Here, we review key biological processes and physical phenomena that strongly influence mountain biodiversity and ecosystems and elucidate their associations with potential mountain EBVs. We identify seven EBVs of highest relevance for tracking and understanding the most critical drivers and responses of mountain biodiversity change. If they are implemented, the selected EBVs will contribute useful information to inform management and policy interventions seeking to halt mountain biodiversity loss and maintain functional mountain ecosystems.